This post isn't so much for photographers - but more for your clients in PR and Advertising - although it may prove useful to those new to the photography profession.
If you work in advertising or public relations, there is a good chance that you send a variety of images to different newspapers, magazines, web sites, and other organisations to promote and market your clients.
From past experience, most PR professionals tend to email their preferred image without resizing it for the intended publication. The downside to this is that you may mistakenly send a web sized image for use in a magazine - meaning either no coverage for your client, or at best, more work for a Picture Editor who will need to chase you for a larger version. Similarly, sending a huge image for use on a web site could cause problems for any bloggers, or others, not familiar with image resizing.
It can be a simple process to make sure the images you are sending out suit the intended publisher. Open the image you have received from a photographer (or taken yourself) in pretty much any editing software (such as PhotoShop or Lightroom), and you can view the image size (it literally is called the 'Image Size' menu option in PhotoShop).
Once viewing the image size, you can easily make it smaller (always start with the largest file size you have and make it smaller - try to refrain from enlarging small files, it isn't always a pretty result).
Use the following PDF to select the correct image size prior to sending to publications (click to open a PDF in a new window).